Method of casting employing self-generated vacuum



Jan. 23, 1968 J. N. REDING ETAL METHOD OF CASTING EMPLOYING SELF-GENERATED VACUUM Filed March 5, 1965 INVENIOR$ JO/PIZIV. Aed/ny I BY Mar v/n R. 602% w 9// em/r United States Patent 3,364,976 METHOD OF CASTING EMPLOYING SELF-GENERATED VACUUM John N. Reding and Marvin R. Bothwell, Midland, Mich.,

assignors to The Dow Chemical Company, Midland,

Mich., a corporation of Delaware Filed Mar. 5, 1965, Ser. No. 437,581 5 Claims. (Cl. 164-63) ABSTRACT OF THE DISCLOSURE A novel process of casting wherein a mold, unvented to the atmosphere but containing at least one mold cavity opening, is at least partially filled with an atmosphere which reacts with the molten metal to be cast therein to produce an oxidized form of the metal in condensed solid form. The mold cavity opening and metal to be cast are brought into contact with each other thereby to provide reaction between the atmosphere in the mold and the metal thus assuring substantially complete vacuum filling of the mold cavity.

This invention relates to the casting of metals and more particularly concerns a novel method of vacuum casting metals, particularly light metals.

Casting of metals, particularly light metals such as magnesium, aluminum and alloys thereof, has been accomplished by one of three methods, namely, gravity feed, pressure feed, and vacuum feed. Difliculties and problems attend the use of each of these methods as practiced heretofore. To illustrate, in mold cavities filled by gravity, it is quite diflicult and almost impossible to fill small crosssectional areas of irregular cavity configurations. In low pressure casting, both venting and the use of large risers are required. High pressure casting sometimes results in porous, poor quality castings. In addition, pressure casting requires expensive equipment and controls. Vacuum casting techniques known in the art eliminate the need for venting, and normally do not produce porous castings. However, as practiced conventionally, vacuum casting requires relatively expensive and complicated equipment to produce, maintain and control the vacuum required for successful casting.

It is clear that a need exists in the art for a simple and economical method for casting metals which incorporates the advantages of conventional vacuum casting, i.e. elimination of vents and the use of large risers, but which also eliminates the need for expensive and complicated pressure or vacuum generating equipment and controls.

It is a principal object of the present invention to provide a simple and economical method for casting metals without the use of expensive pressure and vacuum producing equipment and controls.

It is also an object of the present invention to provide 3,364,976 Patented Jan. 23, 1968 FIGURE 1 illustrates one embodiment of a mold operable in the present process.

FIGURE 2 is a sectional view taken along line 22 of FIGURE 1.

FIGURE 3 shows a mold submerged in a bath of molten metal.

In carrying out the process of the present invention, a mold 10 unvented to the atmosphere, but containing at least one mold cavity opening 12, is provided. The mold cavity 14 is filled at least partially with an atmosphere which reacts with the molten metal 16 to be cast to produce an oxidized form of the metal 16 in condensed solid form. The mold cavity opening 12 and metal 16 to be cast are brought into contact with each other whereupon substantially complete filling of the mold cavity 14 occurs. Operability requires that all vents or openings in the mold 10 that communicate with the atmosphere external to the mold 10 be below the surface of the molten metal 16 being cast when the mold 10 and melt are in contact.

The actual contact time between the metal and mold required to prepare castings by the present novel process will vary from operation to operation dependent on the shape and size of the mold openings and mold cavity to be filled, rate of reactivity between the atmosphere in the mold and metal being cast and the like factors. However, ordinarily sound castings are realized at contact times ranging from 30 seconds or less to about several hours or more and usually from about a minute to about 2 hours. Preferably, in ordinary operations casting times of from about 1 minute to about 1 hour are used. However, contact of the mold with the metal for extended periods of time after casting is complete is not detrimental.

- In practicing one embodiment of the invention, a mold constructed of non-porous material, such as graphite, hard carbon, mild steel, or cast iron with any joined surfaces thereof being relatively tight fitting, and having a predetermined mold cavity configuration, is provided. This mold contains a mold cavity opening of such size in relation to the cavity size that filling can be accomplished therethrough, the cavity opening however being sufliciently small to restrict loss of the molten metal, regardless of the position of the opening relative to the cavity, within a reasonable time after the cast metal starts to cool and solidify. The mold cavity is filled with a gas reactive with the metal to be cast and which produces a condensed solid oxidized form of the metal which has a low vapor 1 pressure at the melt temperature of the molten metal to be cast. The mold ordinarily is heated to a temperature which assures that the metal will remain molten as it enters the mold during the casting operation. The heating can be accomplished by immersing the mold itself in the 3 melt, or by applying heat to the mold by methods known a method for casting light metals without the need of using large and numerous risers other than those required to compensate for normal shrinkage.

It is a further object of the present invention to pro- I use of venting, large riser, or expensive vacuum and gencrating equipment and controls.

These and other objects and advantages readily will become apparent from the detailed description presented hereinafter when read in conjunction with the drawing in which to one skilled in the art. If the mold is immersed into the melt, filling of the cavity occurs as the self-generated vacuum is produced from the reaction between the gas in the cavity and the molten metal with formation of the condensed phase.

An unexpected advantage accompanying this process resides in the fact that if the mold is entirely immersed in the melt, it does not need to be entirely gas or liquid tight.

If the mold is heated externally, the molten metal is transported to the mold to accomplish filling, for example, by a ladle, or a feed pipe. After filling has been accomplished, the heat is removed either by withdrawing the heat source or withdrawing the mold out of the melt bath. After cooling of the mold and solidification of cast metal, the casting in accordance with standard foundry practices, usually is removed from the mold and further cooled.

The only risers necessary when following the practice of this invention are those required to compensate for normal shrinkage upon cooling.

The mold cavity of the mold described directly hereinbefore can be a single hollow chamber or can consist of a mold form having a multiplicity of passages. Additionally, the hollow mold cavity can contain a variety of porous media which are thermally stable and not detrimentally attacked at process temperatures and which have interconnected channels and passages. Examples of suitable mold fillers are compacted particulate materials and powders, felts, wools, sponges, bundles of wires and thin rods, fibers and the like. The use of such fillers provides a ready means for producing metal impregnated compacts, metal filled composite structures, etc.

Particulate fillers which have been found to be suitable for use in the preparation of metal impregnated composites of the light metals such as magnesium include, for example, steel and iron wires, iron and steel felts, iron and steel wools, particulate refractory powders such as Carborundum (silicon carbide), Alundum (aluminum oxide), asbestos fibers, refractory metals (tungsten, molybdenum and tantalum) in the form of wires, fibers, powders and the like, carbon fibers, powdered, particulate and fibrous boron, etc.

In another embodiment of the present invention, an integral porous body of a refractory material, metal or the like material as described hereinbefore having interconnected pores can be used directly as a mold form. In operation with this type mold, the porous material ordinarily is entirely submerged Within the melt for a predetermined period of time. Alternatively, with this latter type mold, the mold member can be fitted with a gas and liquid tight sheath or cover, the bottom or another small portion being left open. The open portion of the mold is immersed into or contacts the melt during casting operations. This embodiment also is a particularly effective means for producing metal impregnated compacts and metal filled composite structures.

One particularly useful composite resulting from this latter embodiment is a magnesium impregnated sponge iron. This material can find utility as an article of manufacture for introducing magnesium smoothly and controllably into ferrous based melts, e.g. for desulfurization and producing nodular iron. A prime advantage of the present composition is that no foreign materials, e.g. carbon or refractory material are introduced into the ferrous melt being treated. With this material, and depending on the porosity of the sponge iron, composites containing 30 percent or more by weight magnesium can be prepared.

The actual gas or vapor atmosphere required within a mold cavity for a given casting operation will depend upon the metal being cast since the mold atmosphere must be a gas or combination of gases and/or vapors which will react with the liquid metal surface of the casting melt to form a condensed, solid, low volatility product, such as the oxide or nitride of the metal being cast. For example, it was found that in casting magnesium into a mold cavity, the reaction of liquid magnesium with oxygen and nitrogen in air present within the mold gives a small quantity of solid magnesium oxide and magnesium nitride and thereby simultaneously generate a vacuum sufficient to cause substantially complete filling of the mold.

If the metal-gas reaction tends to form a protective film of reaction product on the molten metal surface, thereby stopping or impeding the metal-gas reaction, various means known to the art may be used to overcome such protective effect, such as shaking or vibrating the mold. This has the effect of disturbing or breaking the non-gaseous reaction product coating on the metal surface so as to make available fresh surfaces of metal in order that the gas-metal reaction can proceed to generate the needed vacuum. I t

For those metals which form a protective film with the atmosphere within a mold or which are unreactive with mold atmosphere, a small amount of metal reactive with the atmosphere in the mold at the casting melt temperatures can be placed therein. This reactive metal reacts with the atmosphere in the mold to form a condensed, solid reaction product of low volatility thereby providing a substantial vacuum within the mold and allowing the metal desired to be cast to readily penetrate and fill the mold cavity. The amount of the vacuum forming reactant metal to be employed ordinarily is about that required to react with the gas within the mold, although an excess of this metal can be placed in the mold if desired. For most operations, it is understood that this reactive metal and the oxidized product resulting therefrom must not detrimentally affect the properties or other characteristics of the cast product being produced.

The following examples will serve to illustrate further the present invention but are not meant to limit it thereto.

Example 1.-A split graphite mold having a single .mold cavity and fitted with a removable lid was provided. When the two mold members were clamped shut, the

I mold cavity was completely sealed except for a 0.030

inch diameter inlet orifice drilled from the outside into the mold cavity. In the clamped position the mold was first preheated to about 1300 F. and then immersed in molten magnesium at about 1300" F. for about 10 minutes, removed and cooled to solidify the magnesium contained therein. After cooling the mold was opened. The magnesium completely filled the cavity except for the normal shrinkage volume. (If desired this could be compensated for by incorporating a small riser into the mold.)

Example 2.-The procedural steps described for Example 1 were repeated using a shaped cast iron mold, with magnesium as the melt. Examination of the cooled, solidified product indicated that the magnesium substantially filled the mold cavity; only the normal shrinkage volume remained unfilled.

As a control, the empty molds used in Examples 1 and 2, before being again immersed in the melt, were filled with argon, a gas inert to magnesium, by simple displacement of the air in the cavity. Examination of the molds after the immersion treatment indicated there was no filling of the mold cavities by magnesium. This is illustrative of the fact that operability of the present novel process results from a gas-metal reaction forming a condensed -substantially non-volatile product and a reduced pressure, 'or even substantially complete vacuum in the mold.

Example 3.A steel mold having a rectangular 4 in. x 4 in. x in. thick mold cavity and a inch diameter aperture in one end was filled with dried 500 mesh US. Standard Sieve silicon carbide abrasive grains. These were tamped in the mold and the mold cover then fastened in place. The mold and its contents were preheated at about 1300 F. for about 15 minutes. After this time, the mold was completely submerged in a melt of AZ91A magnesium base alloy (ASTM designation) for about 15 minutes, the melt being maintained at about 1300 F. The

5 mold was removed from the melt and cooled. Examination of the mold contents indicated this to be a composite of the magnesium alloy and the silicon carbide. The casting was sound, i.e. had no voids. Examination of a section of the casting at high magnification showed that the magnesium alloy filled the interstices between the grains of abrasive. The composite had a density of 2.50 grams per cubic centimeter. This corresponds to a prod-- uct having about 50 volume percent silicon carbide and about 50 volume persent AZ91A alloy.

Example 4.-The inch diameter by 1% inch long cylindrical mold cavity of a graphite mold was filled with dried, crystalline boron powder having a maximum size of 200 mesh, US. Standard Sieve. This mold had a Mr inch mold cavity opening. The powder was tamped into the mold to obtain tight packing. The mold and its contents were preheated at 1300 F. for 15 minutes and then were immersed in a molten AZ91A magnesium base alloy maintained at about 1300" F. for about 1.5 hours. The mold was removed from the melt and cooled. Examination of the solidified product indicated this to be a sound casting wherein AZ 91A alloy filled substantially all of the interstices of the original boron powder charge.

Example 5.A 1.5 inch diameter by 2 inch long can shaped cylindrical steel mold was filled with dried, minus 200 mesh (U.S. Standard Sieve) aluminum oxide abrasive grains. The mold was then fitted with a cap having a inch mold cavity opening. The mold and contents were preheated at about 1300 F. for about 1 hour and then immersed in molten AZ91A magnesium base alloy at about 1300 F. for about 1 hour. After removal from the melt and solidification, the composite was found to be sound with AZ91A alloy filling substantially all of the powder interstices.

Example 6.227 stainless steel cables each containing 2l0.005 inch diameter wires were compressed into a bundle about inch in diameter by about 2 inches long. An open-ended thin metal tubing, as a mold wall, was then placed around the bundle. About 60 percent by volume of the resulting enclosed bundle was metal. Within each cable, the wires were substantially cylindrically closepacked corresponding to about 90% by volume of each cable occupied by wire. The resulting sheathed cable bundle was preheated at about 1200 F. for about 20 minutes and then totally submerged in molten, commercially pure magnesium at about 1350 F. for about 30 minutes. Examination of a cross-section of the resulting composite indicated that all interstices including those between the cylindrically close-packed wires of the cable strands were filled with magnesium.

Example 7.Fine steel wool was compressed into a tubular /2 inch diameter by 2 inch long steel mold to about 35 percent of its theoretical density. The mold and contents were preheated for about 15 minutes at about 1200 F. and then totally submerged for about 10 minutes in molten magnesium maintained at about 1350 F. After cooling and solidification the resulting composite was found to be sound with magnesium filling substantially all of the interstices in the steel wool.

This run was repeated using a fine graphite felt of about 10 percent theoretical density instead of the steel wool matrix. The felt was not tamped but merely placed in the mold. The mold and its contents were preheated at about 1200 F. for about 30 minutes and then totally submerged in molten magnesium for about 30 minutes at about 1350 F. On cooling and solidification, the composite was found to be sound with magnesium filling substantially all interstices of the felt.

Example 8.-Foundry coke was preheated for about 2 minutes at about 1000 F. and then totally submerged in molten magnesium for about minutes at about 1300 F. After cooling and solidification, it was found upon examination of the resulting composite product that the magnesiimi had filled substantially all of the pores of the coke.

Example 9.An irregularly shaped chunk of sponge iron about 2 inches on a side and having a very fine porosity was preheated at about 1200 F. for about 30 minutes. The heated chunk was totally submerged for about 30 minutes in molten magnesium at about 1350 F. Following this period the chunk of sponge iron was removed from the melt and cooled to solidify the magnesium. Examination of a section of the resulting product showed that magnesium had filled substantially all of the pores of the sponge iron. The sponge iron-magnesium compact contained about 33 percent by weight magnesium. This is equivalent to about 67 percent by volume of magnesium.

Example 10.A bottle-shaped Vycor glass mold was filled with dried, 80 mesh (U.S. Standard Sieve) silicon carbide abrasive grains and the whole preheated at about 1300 F. for about one hour. The bottle-shaped mold was totally submerged in an aluminum base alloy containing about 5 percent magnesium maintained at about 1400 F. for about two hours. The resulting product after cooling and solidification was found to be a sound casting with 6 the aluminum based alloy filling substantially all of the interstices of the particulate silicon carbide matrix.

Example 11.-A steel cylindrical mold having one closed end with a /s inch mold cavity opening and a defining mold cavity of about /2 inch diameter and about 6 inches long was filled with dried silicon car-bide abrasive grains (500 mesh, U.S. Standard Sieve). These were tamped Ito provide a dense porous mass and the open end of the mold then crimped tightly to seal off the mold cavity.

The so-filled mold was preheated to a temperature of about 1700 F. for about 30 minutes. The heated mold was submerged in a melt of AZ91A magnesium base all-0y at about 1300 F. and maintained therein for about one minute.

The mold was removed from the melt and cooled.

The resulting compact upon examination was found to he a substantially void free composite of the magnesium alloy and silicon carbide, the magnesium filling the interstices between the silicon car-bide particles.

The composite had a density of 2.45 grams per cubic centimeter. This is indicative of a casting having about 46 volume percent silicon carbide and about 54 volume percent of the magnesium alloy.

In a manner similar ot that described for the foregoing examples, other metals can be cast into molds containing an atmosphere reactive with the metal being cast to prepare single metal products, composites and the like structures.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that we limit ourselves only as defined in the appended claims.

We claim:

1. A method of casting metals which comprises; providing a molten metal bath, providing a mold, said mold containing at least one mold cavity opening communicating with the mold cavity and the exterior of the mold, said mold cavity containing an atmosphere which is at least partially reactive with a molten metal being cast in said mold, said atmosphere forming a condensed solid oxidized form of said metal by reaction with said metal, contacting the mold cavity opening with said molten metal while maintaining all openings of the mold which communicate with the atmosphere external to the mold below the surface of the molten metal bath being cast, maintaining said mold opening and said molten metal in contact for a predetermined period of time, causing said mold atmosphere to react with said molten metal thereby to produce a condensed solid oxidized form of said metal and to :genenate a vacuum in said mold sutlicient to cause substantially complete filling of the mold with said molten metal separating the mold from contact with the molten metal bath and cooling and solidifying the resulting metal product cast in said mold.

2. The process as defined in claim 1 land including the step of submenging the mold containing the reactive atmosphere in a molten metal to be cast in said mold.

3. The process as defined in claim 1 and including the step of preheating the mold to a temperature about that of the metal to be cast therein prior to contact'ng said mold and said molten metal to be cast therein.

4. The process as defined in claim 1 wherein the mold cavity is a single chamber and there is only one mold cavity opening in said mold.

5. The process as defined in claim 1 for providing a metal filled composite structure and including the step of introducing a porous medium having interconnected channels and passages containing an atmosphere which is at least partially reactive with a molten metal being cast in said mold into said mold cavity prior to contacting said mold and said molten metal to be cast, said porous medium being thermally stable and not detrimentally attacked at process temperatures.

7 References Cited 2,671,955 2,192,792 UNITED STATES PATENTS 2,665,999 2/1966 Hucke 22-202 X 3 1 415 2/1913 Scctt et a1 22202 X 5 10/1925 Hunt 29191.2 4/1959 Bergh 75-53 8 Grubel et a1 29182.1 X Kurtz 29182.1 Koehring 29182.1 X Conant 72-,202

I. SPENCER OVERHOLSER, Primary Examiner.

V. K. RISING, Assistant Examiner. 

